66. Sure, if that were a word he would be that.

A mistress is a long-term female lover and companion who is not married to her partner

...check.

the term is used especially when her partner is married

...check.

the relationship generally is stable and at least semi-permanent

...check.

however, the couple does not live together openly

...check

Also the relationship is usually, but not always, secret.

...until recently, check

There is an implication that a mistress may be "kept"—i.e., that the lover is paying for some of the woman's living expenses.

...unclear at this point. Given his power (and the fact that she made a lot of money by gaining access to details of his life) being his biographer may count in this regard. Either way this is an implication and popular usage rather than an explicit requirement.

So . . . were you trying to prove my point for me? Because that really isn't necessary.